Prokaryotic and eukaryotic cellsEdexcel A-Level Biology B: Subtopic test

1. 1
   A hospital laboratory identifies two species of bacteria from patient samples. Species 1 stains purple with the Gram stain and species 2 stains pink. Penicillin, which stops the cross-links in peptidoglycan from forming as a bacterial cell wall is built, kills species 1 at a low concentration, but species 2 is affected only at a much higher concentration.

   (a)

   Which row correctly describes species 1?

   [1 mark]

   • AGram-negative, with a thin peptidoglycan wall and an outer membrane
   • BGram-negative, with a thick peptidoglycan wall and no outer membrane
   • CGram-positive, with a thin peptidoglycan wall and an outer membrane
   • DGram-positive, with a thick peptidoglycan wall and no outer membrane

   (b)

   Penicillin does not damage human cells at the concentrations used. Which statement best explains this?

   [1 mark]

   • AHuman cells have cell walls made of chitin that penicillin cannot cross
   • BHuman cells have no peptidoglycan cell wall, so penicillin has no target
   • CHuman cells have a nucleoid, which protects them
   • DHuman cells have 70S ribosomes, which penicillin cannot affect

   (c)

   Suggest why species 2 is much less affected by penicillin than species 1.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student observes human cheek cells stained with methylene blue under a light microscope. She then looks at a transmission electron micrograph (TEM) of a similar cell in a textbook. In the TEM, mitochondria show their internal folds and ribosomes appear as tiny dark dots, but neither is clear in the light microscope.

   (a)

   Why does the TEM show ribosomes when the light microscope does not?

   [1 mark]

   • AElectrons have a much shorter wavelength than light, so the TEM has a much higher resolution
   • BThe TEM has a higher magnification, and higher magnification always gives higher resolution
   • CRibosomes are only present in cells that are prepared for the TEM
   • DThe TEM can be used with living cells, in which ribosomes are active

   (b)

   Why did the student stain the cheek cells with methylene blue?

   [1 mark]

   • ATo kill the cells so they stop moving
   • BTo increase the magnification of the cells
   • CTo increase contrast, because most cell structures are almost transparent
   • DTo make the specimen thinner so that light passes through it

   (c)

   The image of one mitochondrion in the TEM is 45 mm long. The magnification of the image is ×15 000. Calculate the actual length of the mitochondrion in micrometres (µm).

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student is using a light microscope to measure cheek cells. A stage micrometer is a slide with a scale 1 mm long divided into 100 equal divisions. Using the ×40 objective, she finds that 4 divisions of the stage micrometer line up exactly with 25 divisions of the eyepiece graticule. She then finds that one cheek cell spans 35 eyepiece divisions.

   (a)

   Calculate the actual length of the cheek cell in micrometres (µm). Show your working.

   [3 marks]

   (b)

   Explain why the eyepiece graticule must be calibrated again when the objective lens is changed, and state two features of a good drawing of the cells.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A pancreatic acinar cell makes and secretes large quantities of digestive enzymes, which are proteins. Under the electron microscope its cytoplasm shows extensive rough endoplasmic reticulum, a prominent Golgi apparatus, many mitochondria and a large nucleus with a dark nucleolus. The bacterium Escherichia coli is also viewed under the electron microscope.

   (a)

   Compare the ultrastructure of the bacterium with that of the pancreatic cell.

   [6 marks]

   (b)

   Explain how the organelles in the pancreatic cell are involved in making and secreting the digestive enzymes, and why the cell has many mitochondria.

   [6 marks]

   Total for question 4: 12 marks

End of questions